Semiconductors are materials which exhibit a band gap between the material's valence and conduction bands, typically no more than a few eV. Because this energy gap is so low, as the temperature of the crystal is increased, electrons are thermally excited and easily from the valance band to the conduction band. The electrical properties of these materials, therefore, are effected not only by the movement of electrons into the conduction band but also by the formation of vacant sites or "holes" in the valence bands left behind by the departing electrons. Both can conduct current.
Holes also may be created by the interaction of energetic radiation, such as X-rays, gamma rays, and the like, with intrinsic semiconductors and, therefore, one should be able to use these materials as detectors for measuring high energy radiation. In fact, high-resistivity semiconductor radiation detectors are widely used for detecting ionizing radiation due to their ability to operate at room temperature, their small size and durability. Such detectors are used in a wide variety of applications, including medical diagnostic imaging, nuclear waste monitoring, industrial process monitoring, and space astronomy. Ionizing radiation includes both particulate radiation, such as alpha or beta particles, and electromagnetic radiation, such as gamma or x rays.
If all the electrons and holes generated by the ionizing radiation reach their respective electrodes (i.e., the electrons reach the anode and the holes reach the cathode), the output charge signal will exactly equal the charge from the energy deposited within the crystal by the radiation. Because the deposited charge is directly proportional to the energy of the ionizing radiation, the semiconductor detector provides a means for measuring the energy of the ionizing radiation.
Room temperature detectors, however, suffer from a serious drawback. Because of limitation in the transport properties of the bulk semiconductor crystal, some of the electrons and, more particularly, some holes are generally lost by being trapped as they move toward the respective electrodes under the influence of the external electrical field. This is particularly evident for semiconductors wherein the transport properties of one carrier type (e.g., electrons) are much better than those of another type (in this example the "holes"). Under such circumstance, therefore, the amplitude of the output charge signal becomes dependent on the position within the crystal at which the ionizing radiation is absorbed. Generally speaking, the amplitude is less than the charge deposited by the ionizing radiation and results in a corresponding reduction of energy measurement accuracy, poor resolution, and reduced peak efficiency.
This loss (or trapping) of charge in a radiation detector results in distorted and asymmetrical spectral peak shapes known as "hole tailing."
The inability to eliminate "hole" drift current is a major impediment for the use of room temperature semiconductors as detectors. Gamma-ray spectroscopy is particularly encumbered because pulse height spectra produced by these devices are distorted by this process in a way best explained by the illustration in FIG. 1. Here, mono-energetic gamma rays 1-3 interact with positions 4-6 producing charge signal responses of different height because the total distance drifted by each charge is different in all three cases. This phenomenon is well known in the prior art and has been described by many researchers. It is widely understood to be the major deficiency limiting the effectiveness of room temperature semi-conductor materials, such as Cd.sub.1-x Zn.sub.x Te and Hgl.sub.2, used as gamma-ray detectors.
This invention, therefore, pertains generally to a method for constructing a simple room-temperature, semiconductor device for detecting gamma radiation. More particularly, this invention pertains to a method and an apparatus which completely, or nearly completely, suppresses spurious signal currents arising from hole drift or "hole-tailing."